My family will be visiting UK in August for a few weeks and I'm thinking about taking them for a side trip to Istanbul.
The airfare between LHR and Istanbul is very cheap right now (around 50 quid one way + tax). Is this the norm between the two cities? In other words, I don't need to jump on booking the tickets until the dates are a bit closer, do I?
I'd actually say those are very good prices and if you're sure about the airline and quite firm on going and can set dates, go for it. You will want a direct flight.
Last time we did this flight we managed to do so at the same time as the Pope--which meant SIX airport checks to get in and out. Istanbul is also one of the cities that you will want to let your credit cards/bank card know you are visiting as they will shut you off.